Control.Monad.Reader.Class

Computation type:

Computations which read values from a shared environment.

Binding strategy:

Monad values are functions from the environment to a value. The bound function is applied to the bound value, and both have access to the shared environment.

Useful for:

Maintaining variable bindings, or other shared environment.

Zero and plus:

None.

Example type:

Reader [(String,Value)] a

The Reader monad (also called the Environment monad). Represents a computation, which can read values from a shared environment, pass values from function to function, and execute sub-computations in a modified environment. Using Reader monad for such computations is often clearer and easier than using the State monad.

Inspired by the paper Functional Programming with Overloading and Higher-Order Polymorphism, Mark P Jones (http://web.cecs.pdx.edu/~mpj/) Advanced School of Functional Programming, 1995.

Documentation

class Monad m => MonadReader r m | m -> r where

See examples in Control.Monad.Reader. Note, the partially applied function type (->) r is a simple reader monad. See the instance declaration below.

Minimal complete definition

(ask | reader), local

Methods

ask :: m r

Retrieves the monad environment.

local

Arguments

:: (r -> r)

The function to modify the environment.

-> m a

Reader to run in the modified environment.

-> m a

Executes a computation in a modified environment.

reader

Arguments

:: (r -> a)

The selector function to apply to the environment.

-> m a

Retrieves a function of the current environment.

Instances

MonadReader r m => MonadReader r (MaybeT m)

Methods

ask :: MaybeT m r

local :: (r -> r) -> MaybeT m a -> MaybeT m a

reader :: (r -> a) -> MaybeT m a

MonadReader r m => MonadReader r (ListT m)

Methods

ask :: ListT m r

local :: (r -> r) -> ListT m a -> ListT m a

reader :: (r -> a) -> ListT m a

(Monoid w, MonadReader r m) => MonadReader r (WriterT w m)

Methods

ask :: WriterT w m r

local :: (r -> r) -> WriterT w m a -> WriterT w m a

reader :: (r -> a) -> WriterT w m a

(Monoid w, MonadReader r m) => MonadReader r (WriterT w m)

Methods

ask :: WriterT w m r

local :: (r -> r) -> WriterT w m a -> WriterT w m a

reader :: (r -> a) -> WriterT w m a

MonadReader r m => MonadReader r (StateT s m)

Methods

ask :: StateT s m r

local :: (r -> r) -> StateT s m a -> StateT s m a

reader :: (r -> a) -> StateT s m a

MonadReader r m => MonadReader r (StateT s m)

Methods

ask :: StateT s m r

local :: (r -> r) -> StateT s m a -> StateT s m a

reader :: (r -> a) -> StateT s m a

MonadReader r m => MonadReader r (IdentityT m)

Methods

ask :: IdentityT m r

local :: (r -> r) -> IdentityT m a -> IdentityT m a

reader :: (r -> a) -> IdentityT m a

MonadReader r m => MonadReader r (ExceptT e m)

Since: 2.2

Methods

ask :: ExceptT e m r

local :: (r -> r) -> ExceptT e m a -> ExceptT e m a

reader :: (r -> a) -> ExceptT e m a

(Error e, MonadReader r m) => MonadReader r (ErrorT e m)

Methods

ask :: ErrorT e m r

local :: (r -> r) -> ErrorT e m a -> ErrorT e m a

reader :: (r -> a) -> ErrorT e m a

MonadReader r' m => MonadReader r' (ContT r m)

Methods

ask :: ContT r m r'

local :: (r' -> r') -> ContT r m a -> ContT r m a

reader :: (r' -> a) -> ContT r m a

Monad m => MonadReader r (ReaderT r m)

Methods

ask :: ReaderT r m r

local :: (r -> r) -> ReaderT r m a -> ReaderT r m a

reader :: (r -> a) -> ReaderT r m a

MonadReader r ((->) r :: * -> *)

Methods

ask :: r -> r

local :: (r -> r) -> (r -> a) -> r -> a

reader :: (r -> a) -> r -> a

(Monad m, Monoid w) => MonadReader r (RWST r w s m)

Methods

ask :: RWST r w s m r

local :: (r -> r) -> RWST r w s m a -> RWST r w s m a

reader :: (r -> a) -> RWST r w s m a

(Monad m, Monoid w) => MonadReader r (RWST r w s m)

Methods

ask :: RWST r w s m r

local :: (r -> r) -> RWST r w s m a -> RWST r w s m a

reader :: (r -> a) -> RWST r w s m a

asks

Arguments

:: MonadReader r m

=> (r -> a)

The selector function to apply to the environment.

-> m a

Retrieves a function of the current environment.